1. Field of the Invention
The present invention relates particularly to a control system for controlling the ignition timing of a spark-ignition internal combustion engine, having a crankshaft, and more particularly to an improved electronic ignition control circuit for advancing or retarding the ignition timing in response to the rotational speed of the crankshaft.
2. Description of the Prior Art
The conventional ignition timing control systems of the kind comprise a centrifugal spark advance mechanism for adjusting the ignition timing of an engine according to the number of revolutions of the engine and a vacuum control mechanism adapted to advance or retard the ignition timing according to the engine intake manifold vacuum. This is based on the concept that primarily the ignition of an engine can be accurately timed to a satisfactory degree according to two factors: the engine revolutions and engine intake manifold vacuum.
With the conventional ignition timing control systems described above, a spark advance characteristic curve corresponding to the engine rpm is obtained by means of the centrifugal spark advance mechanism and then a parallel displacement of this spark advance characteristic curve is effected by the vacuum control mechanism according to the engine intake manifold vacuum so as to obtain a desired ignition timing.
Furthermore, when the conventional ignition timing control system is used in race boats, race cars and other applications of the internal combustion engine it utilizes a distributor, a magneto or a magnetic or optical device to monitor the position of the crankshaft directly. The system that uses the distributor for the timing function is capable of varying the amount of timing advance and does so by a mechanical device involving the use of a transducer, such as an electrical transducer, a mechanical transducer or a vacuum transducer. None of these transducers provides to a high degree accurate control of the advance function; and none of these transducers provide flexibility in the type of advance characteristics that may be obtained.
Presently, timing systems that employ computers to modify the advance characteristics of the ignition timing are made less accurate by the fact that they still use transducers to modify the spark timing function. A timing system utilizing a distributor to obtain its basic timing function also utilizes mechanical points, magnetic proximity detectors or optical pick-off devices. This timing system employs a mechanical device to vary the spark timing. There is a disadvantage in using the distributor to obtain the basic timing function in that there is a significant error between the position of the distributor shaft and the position of the crankshaft. The distributor's position error is caused by the slack between the crankshaft, camshaft and the distributor shaft. Since the position of the crankshaft is the important factor in timing the spark for optimum horsepower, optimum efficiency, minimum exhaust emissions or whatever engine characteristic is most desirable for the particular application for which it is being used it is desirable to eliminate or reduce the distributor's position error relative to the crankshaft.
A small number of timing systems presently in use actually "time" directly off the crankshaft, using it as a basic timing reference device by using a magnetic or an optical device. While this eliminates the error in the basic timing function, it renders the normal advance-modification techniques useless.
There are also mechanical timing systems that employ a centrifugal device and/or a vacuum control device, but these systems are cumbersome and exhibit poor accuracy, thereby defeating any advantage that may be gained by employing the crankshaft itself as the basic timing reference. Furthermore, these timing systems offer very little flexibility in the spark advance characteristics that may be obtained.
There are some devices in the prior art that obtain a spark timing which is accurate to a high degree and that provide flexibility in the spark advance characteristics that may be obtained. For example, U.S. Pat. No. 3,871,342 entitled Electronic Ignition Timing Control Circuit For Internal Combustion Engine, issued to Hiroshi Fujinami and Katuyuki Takagi on Mar. 18, 1975 teaches an electronic timing control circuit that uses a digital device primarily in its computing circuits to provide stability against variations in both voltages of the power supply of the control circuit and the ambient temperature. This control circuit is too cumbersome to provide the most effective and reliable device to achieve an optimum timing function. Furthermore, this control circuit requires, in addition to reference angular detecting device, a separate revolution detecting device.
U.S. Pat. No. 3,768,451, entitled Ignition Timing Control System, issued to Hisaji Okamoto on Oct. 30, 1973, teaches an ignition timing control system having a set of ignition times present to meet the ignition timing requirement of an engine operating under various satisfactory operating conditions, whereby an ignition time which meets the ignition timing requirement of the engine at each time is selected from the set of ignition times to be applied to the engine according to the engine revolutions, engine intake manifold vacuum, and temperature of the engine cooling water. This ignition control system is too cumbersome to be practical.
U.S. Pat. No. 3,853,103, entitled Ignition Timing Control System for Internal Combustion Engine Ignition Systems, issued to Joseph Wahl and Wolf Wessel on Dec. 10, 1974, teaches an ignition timing control system having a pulse generator which provides a pulse train representative of angular position of the engine crankshaft, and a marker pulse, at a predetermined angular crankshaft position. A counter is connected to the train of pulses to start counting upon occurrence of the marker pulses. A digital/analog converter converts the binary count numbers into an analog signal, which is compared with engine operation signals representative of spark advance or retardation, the comparator providing an output when the count derived from the counter and the operation parameter control signals match. The engine operation control signal may be a composite of signals commanding spark advance or spark retardation, such as speed signals, load signals or other operating parameter signals, applied to the comparator as varying voltages or currents. This design is too cumbersome to be practical.